I agree with Alchemy. It's much easier to go to Tenerife and let yourself be carried across by the tradewinds. That part of the trip will be a pleasant run.
But, you might say, the distance for that leg is about the same as a Poole-Halifax- 2500nm! So why go for a total journey of about 5700 nm, port to port? At five knots, continuously, without ever stopping, it will last about 48 days.
That seems like a long time, but if you were to take the rhumb line, in all likelihood you will be head
to wind and tacking. Then the duration will be the same. As we all know, compared to a run, tacking is twice the distance, three times the time and four times the hassle. Unless you don't get westerlies, but that seems unlikely.
So to me, it's clearly the southern route. You'll be just as far from shelter if you were to take rhumb line, but there are big differences: the weather
plus there are many cruisers doing that route at this time of the year. It provides company and a sense of security
. On the rhumb line you will meet large commercial
vessels and pray that they see you. Few ships maintain a full-time watch in that area, in spite of Solas regulations
A couple of additional notes: The northern route is rough, even in summer. In the western portion, there are many icebergs, notably growlers, partially submerged containers and the weather
is cold, damp and foggy. You can come across severe lows and worry a lot. I found navigating in that area highly unpleasant. If you are still not convinced about the merits of the southern route, take a look at Pilgrim's blog, referred to above.
I've crossed Biscay many times, it's usually rough but better in the summer but it's a wild place in winter. Be particularly alert, the weather can change quickly and when it does, you'll get a strong south-westerly wind which later veers to NW. If you are south of 45* N, going east, inside Biscay, to seek shelter is a very bad idea, it's a trap with no way out. Best turn back and head
for La Rochelle or grin and bear it and head for Santander or la Coruna.
Last, if you have the time, you should visit the wonderful Spanish Rias, south of La Coruna. Well worth a visit.